Ladders formed of extruded aluminum, fibreglass or composite materials and comprising a pair of open C-shaped side rails and cylindrical or non-cylindrical shaped rungs are known. In the manufacture of such ladders having cylindrical rungs, each rung is inserted into round holes which have been drilled or stamped out of the side rail, and swaged therein for retention.
A major drawback of ladders having cylindrical rungs is that the rungs tend, upon continuous use, to rotate within the hole of the side rail. Another drawback is that the curved step surface of the cylindrical rung is conducive to allow of slipping by the climber and, thus, requires more attention when the climber is climbing up or merely standing on the rung.
To overcome these safety drawbacks of cylindrical rungs, non-cylindrically shaped rungs have been developed. One preferred form is the D-shaped rung, known as the D-rung. In a D-rung ladder, the rungs are set into a D-shaped aperture in the side rail with the flat side of the rung positioned at an angle, typically 15.degree., to the horizontal plane of the ladder when the ladder is held upright, such that when the ladder is leaned against a vertical surface, the flat or step side of the D-rung is substantially horizontal and parallel to the ground. In addition, this type of rung provides more safety in that it cannot rotate within its hole in the side rail and provides more comfort to a person standing on the rung for an extended period of time.
It will be appreciated that non-round holes cannot be drilled in ladder side rails and are generally made by stamping or punching out the desired shape in the C-side rail by means of a punch press assembly.
Extruded aluminum ladders having side rails of hollow, closed, generally rectangular form are known as box ladders. Within the term "box ladder", is also included, for example, side rails of a square, round or oval cross-sectional form. These box ladders, because of their greater inherent strength, are preferred over C-side rail ladders.
While it is relatively straight forward to produce round holes in box side rails by drilling, it is more difficult to economically provide non-round holes along the length of the box side rail by stamping, punching and the like, without some deformation of the box, even with use of a mandrel. Further, use of a mandrel in a stamping operation to stamp out holes in box side rails becomes increasingly difficult as the length of the side rail increases. Increased mandrel weight considerations, high tolerance requirements and physical manipulations of the punch press, dies and the like, make the manufacture of box side rails having non-round holes economically unattractive for ladder lengths greater than 6 feet. For these reasons, the acceptance of the more safer, non-cylindrical rung ladders, particularly the D-rung ladder, has been reduced.
It is an object of the present invention to provide a box side rail ladder having non-cylindrical rungs.
It is a further object of the invention to provide an improved method of manufacturing a box side rail ladder.
It is yet a further object of the invention to provide a method of manufacturing a box side rail ladder having non-cylindrical rungs.
In its simplest aspect, the invention provides a method of manufacture of cylindrical and non-cylindrical rung box ladders from the assembly of a pair of box side rails and rungs wherein each box side rail is assembled from two lengths of section members each having pre-formed holes intermittent along their lengths.
Accordingly, in its broadest aspect, the invention provides a method of manufacturing a box rail ladder having a first rectangular side rail, a second rectangular side rail and a plurality of rungs, which method comprises forming a series of holes intermittent along the length at pre-determined intervals of a first C-shaped section member;
forming a series of holes intermittent along the length at pre-determined intervals of a second C-shaped section member;
mating said first and second section members to form said first rectangular side rail;
mating said first and second section members to form said second rectangular side rail; and
mating said first and second side rails with said rungs to form said box rail ladder.
Preferably, the invention provides a method of manufacturing a box rail ladder as hereinabove defined wherein said rungs and said holes are non-cylindrical and, more preferably, D-shaped.
The first and second elongated C-shaped section members are generally made of a composite material, fibreglass or of aluminum extruded into appropriate lengths.
The invention, thus, generally provides a method of manufacture of a box ladder having a pair of box side rails, which side rails are formed from two halves of a side rail wherein each of the halves is pre-punched before assembly and which, upon assembly, form a substantially rectangular box section with holes, preferably, non-round holes, to receive the rungs; and box ladders made thereby.
In preferred forms, any non-cylindrically shaped rung having a shape which interferes with rotation of the rung in the side rail hole may be of use in the present invention. However, regard should be had to the utility of such a shape as a rung of a ladder.
A shaped rung providing a flat upper surface to the feet of a user, and, optionally, provided with serrations, is preferred. Examples of preferred rung shapes are triangular-shaped, square-shaped, rhomboidal-shaped and D-shaped rungs.
In the method according to the invention, the holes in a pair of mateable section members are preferably formed by stamping or punching out the members simultaneously. This operation can be readily achieved by positioning the pair of members in a "back-to-back" position wherein the flat body portion of the members fully abut each other. This feature allows for the use of a single punching action for each pair of corresponding holes and, thus, a reduction in capital equipment cost, and a better series of hole alignments.
Each pair of corresponding holes may be formed simultaneously as the other pairs of holes by a stamping operation along the lengths of the pair of abuting section members; but, preferably, each pair of corresponding holes are formed in individual stamping operations.
Thus, it can be seen that the above method provides for the relatively simple and economic manufacture of significant lengths of box ladder without the problem of deformation effected by stamping of a pre-formed rectangular box side rail.
In a further aspect, the invention provides cylindrical and non-cylindrical rung box ladders comprising a pair of box side rails and a plurality of cylindrical or non-cylindrical rungs wherein said box side rails are each formed of a first section member and a second section member engaged therewith.